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The ww Language Specification

Version 0.1 (2026-06-21). Status: normative-intent, in progress.

ww is a systems language: Hare semantics + CSP concurrency, no garbage collector, with a Rob-Pike / Plan-9 sensibility and Go-style explicit package/import. This document is the language contract. Where the two compiler stages disagree with this text, the text is wrong and is a bug to be filed (CLAUDE.md rule 10 requires the stages to agree with each other; this spec requires them to agree with it).

Two implementations track this spec and must stay byte-identical (rule 10): the C bootstrap frontend cmd/wcc (the reference) and the ww reimplementation lib/ww/syntax. The token table in §2 is kept numerically identical between them on purpose.

This is not a tutorial. It is terse by design, modelled on the Go language specification (golang.org/ref/spec): EBNF where a grammar is worth pinning, prose for semantics, and a normative appendix (§12) that consolidates every sanctioned divergence from Hare. The appendix is the highest-value part of this document — it is the single normative home for rules currently scattered across CLAUDE.md, .ai/, and project memory.

Sections marked (reserved) describe surface that is tokenised but not yet given meaning; they are written when the feature lands.

1. Notation

The grammar uses Extended Backus-Naur Form, following the Go spec's conventions:

Production  = production_name "=" [ Expression ] "." .
Expression  = Term { "|" Term } .
Term        = Factor { Factor } .
Factor      = production_name | token | Group | Option | Repetition .
Group       = "(" Expression ")" .
Option      = "[" Expression "]" .
Repetition  = "{" Expression "}" .

| is alternation, {} is zero-or-more, [] is zero-or-one, () is grouping. Terminal tokens are in "quotes" or are the named token classes from §2 (ident, int_lit, …).

2. Lexical elements

2.1 Source representation

Source is UTF-8. The lexer operates on bytes; non-ASCII bytes are legal only inside string and rune literals and comments.

2.2 Comments

Line comments only, introduced by // and running to end of line. There are no block comments (Hare lineage; /* */ is not recognised).

Two comment forms are driver directives, not ordinary comments — they carry module-bundle structure and lex to dedicated tokens:

//ww:module <dotted.path> — set the mangling module path for the decls that follow (token MODPATH).
//ww:module-reset — reset the current module to the empty string before a package-less bundled file (token MODRESET).

These exist for the build driver's source bundling and are not written by hand in ordinary source.

2.3 Tokens

Tokens are identifiers, keywords, operators/punctuation, and literals. Whitespace (space, tab, CR, LF) separates tokens and is otherwise ignored. A semicolon ; terminates declarations and statements and is written explicitly (no automatic insertion).

2.4 Keywords

The following 30 identifiers are reserved and may not be used as ordinary identifiers:

as       break    case     chan     const    continue
def      defer    else     enum     export   false
fn       for      if       is       import   let
match    nil      package  proc     return   static
struct   switch   true     type     void     yield

chan and proc are reserved for the concurrency surface (§11, reserved). package is the package-clause keyword; import introduces an import.

2.5 Operators and punctuation

Assignment:  =  +=  -=  *=  /=  %=  &=  |=  ^=  <<=  >>=
Arithmetic:  +  -  *  /  %
Bitwise:     &  |  ^  ~  <<  >>
Comparison:  ==  !=  <  <=  >  >=
Logical:     &&  ||  !
Grouping:    (  )  {  }  [  ]
Punctuation: ,  ;  :  .  ..  ...  @  ?
Arrows:      <-  ->  =>

<- (channel send/receive) and -> are tokenised but reserved (§11). => separates a match arm pattern from its body. ? and ! are the error operators (§9). : introduces a type ascription / cast (§8) and a field/binding type. .. is the range operator (for range form, §7); ... is the variadic / spread marker (§6.3). @ introduces an attribute (§5.7).

2.6 Identifiers

ident = letter { letter | digit } .
letter = "A"…"Z" | "a"…"z" | "_" .
digit  = "0"…"9" .

A lone _ is the discard identifier: legal as a binding name to mean "ignore this", never as a reference.

2.7 Literals

int_lit   = decimal | hex | octal | binary [ suffix ] .
float_lit = digits "." digits [ exponent ] [ suffix ] .
rune_lit  = "'" ( byte | escape ) "'" .
str_lit   = '"' { byte | escape } '"' .
bool_lit  = "true" | "false" .
nil_lit   = "nil" .

int_lit is held as a u64. nil is the null pointer value (§3.6).
A numeric literal may carry a type suffix naming its type directly: 42u8, 0u64, 1i64, 3.0f32. Suffixes are the fixed-width type names (i8 i16 i32 i64 u8 u16 u32 u64 f32 f64). Without a suffix a literal is an untyped constant (§3.5) whose type is inferred from context.
Escapes in rune and string literals: \\ \" \' \n \t \r \0 and \xNN (two hex digits). Rune literals additionally accept the Unicode escapes \uHHHH and \UHHHHHHHH, yielding a codepoint.

3. Types

A type describes the layout and operations of a value. Every type has a size and an alignment, both queryable with size(T) (§8). No size is ever written as a bare literal in size-computing code — it is routed through the type table (CLAUDE.md rule 13).

3.1 Boolean and void

bool (1 byte; true/false). void (size 0) is the empty type, used as a fn result and as the second arm of a nullable pointer (§3.6).

3.2 Numeric types

Fixed-width integers: i8 i16 i32 i64 (signed), u8 u16 u32 u64 (unsigned). Floats: f32 f64.

Machine-word types (8 bytes on amd64): int, uint, uintptr, size. int and uint are a full machine word — Go-style, not Hare's 32-bit int. The limit constants (INT_MIN, INT_MAX, UINT_MAX) are derived from size(int), never hardcoded to a width (§12).

rune is a 32-bit Unicode codepoint (4 bytes).

3.3 String and slice

str and []T (slice of T) are both 24-byte three-word headers: { ptr, len, cap }. Their .ptr, .len, and .cap pseudo-fields are readable (§8.4). A str is UTF-8 bytes; reinterpreting []u8 as str is strings.frombytes (a pure cast — there is no validation; §12).

Array: [N]T is N elements inline (size N * size(T)). [_]T infers N from the initialiser's element count.

3.4 Aggregate and named types

type S = struct { f0: T0, f1: T1, … };   // struct
type Name = T;                            // named type / alias

struct — named fields laid out in declaration order with natural alignment padding. Duplicate field names are rejected.
tuple — (T0, T1, …). Each element occupies an 8-byte slot (size((u32, u32)) == 16), diverging from Hare's packed tuples (§12).
enum — enum i32 { A = 0, B, … } (a named integer type with named constants) or bare enum { … }.
A named type / alias introduces a new spelling for an underlying type.

TODO(spec): structs carry an internal packed property (no padding); surface its syntax (an @packed attribute? a struct modifier?) once confirmed against cmd/wcc. Likewise pin the static let qualifier spelling and whether static applies outside function bodies.

3.5 Untyped constants

A literal without a suffix has an untyped type (untyped int, untyped float, untyped str, untyped rune, untyped bool, untyped nil). An untyped constant takes a concrete type from its use context (assignment target, operand of a typed operation, fn argument). This is the inference that lets let x: u8 = 5; work without a suffix.

3.6 Pointers and nullability

*T is a pointer to T. A plain *T is non-null; dereferencing it is unconditional and free. A nullable pointer is written as the tagged union (*T | void); nil inhabits it. This is a one-flag fold, zero extra cost for the non-null case, and it is the only optionality mechanism — there is no separate nullable qualifier keyword (§12). ww has no runtime null-check net, so a non-null *T carries the no-null obligation the way C does.

3.7 Tagged unions and the error tag

TaggedType = "(" Type { "|" Type } ")" .

A tagged union (A | B | …) holds one of its variants plus a runtime tag. Variants are discriminated with match (§7) or tested with is and narrowed with as (§8). A variant may carry the error mark !T, which participates in the error model (§9): (i64 | invalid | overflow) is a typical fallible result.

3.8 Function and channel types

fn(P0, P1, …) R is a function type. chan T is a channel of T (reserved, §11). never is the bottom type of an expression that does not return (e.g. a call to abort).

4. Packages and imports

SourceFile = PackageClause { ImportDecl } { TopDecl } .
PackageClause = "package" ident ";" .
ImportDecl    = "import" ( ImportPath | ImportName ImportPath ) ";" .
ImportName    = ident .
ImportPath    = ident { "." ident } .

Every source file begins with a package clause. Within one semantic source action, eligible .ww files share one declared package name. Ordinary production and same-package test sources use p; external test sources may use the related p_test, and the actions remain separate even though one canonical directory owns their test product. The declared name need not equal the directory name or the final component of its canonical import identity.
Directory source eligibility uses Go 1.26.5 filename suffix semantics for WW's fixed linux/amd64 target. In the basename stem before the first dot, a final _test token is ignored for platform matching. A final known OS or architecture token must match linux or amd64; a final known OS followed by a known architecture takes precedence and both must match. The known-name sets are Go 1.26.5's syslist.KnownOS and syslist.KnownArch. Unknown or misplaced suffixes are ordinary, and a platform word without a nonempty underscore prefix is ordinary (linux.ww and plan9_test.ww are selected; x_windows.ww and x_plan9_test.ww are not). Leading-dot/underscore entries are ignored. Eligible names are byte-sorted before source validation. Production excludes selected *_test.ww; test variants classify only those selected test files. An excluded file contributes no declarations, imports, filename collision, package edge, action, export, artifact, initialization, test, or persistent invalidation. After eligibility, two distinct selected basenames in one canonical directory that are equal under Go 1.26.5 Unicode simple folding are rejected after the coordinator's required package-clause classification and production @test validation parses, but before the delegated graph-import scan or tools. An ordinary build compares production names only; one test product compares its production, same-package test, and external-test selections without merging their units.
import acme.codec; loads the canonical package acme.codec. If that package declares package wire;, the importing file sees its exported names as wire.Name; codec.Name is not an additional binding. An explicit alias replaces only that visible qualifier: import stable acme.codec; exposes stable.Name, not wire.Name or codec.Name. Both kinds of binding are scoped to that source file. A sibling file must declare its own import. Neither form exposes an imported declaration as a bare Name; ordinary unqualified lookup remains limited to lexical, builtin, and same-package declarations.
import _ acme.codec; is a blank side-effect import. The lone _ creates no qualifier, exposes no bare declaration, and is never diagnosed as unused. It is nevertheless a real import occurrence: resolution and all missing, self-import, cycle, internal, vendor, and imported-command checks use the dotted path, and the dependency participates in executable and test package initialization. Repeated blank occurrences and blank plus default/explicit named occurrences of one path are valid; every named occurrence remains independently subject to duplicate-binding and unused checks. Each semantic action's package dependency graph is the sorted, deduplicated union of real imports in that action's eligible source set. Occurrences retain their owning file and position, but equal canonical targets create one graph edge. Test-only occurrences never enter ordinary production. Self-import is rejected, except that toolchain-owned external-test self wiring is rebound to the effective augmented package action after ordinary per-site validation.
Canonical import identity is exact and case-sensitive. After contextual local or vendor expansion, two distinct effective identities that are equal under Go 1.26.5 simple folding are a request-wide structural error. Folding is only a temporary collision key: it never changes lookup, action identity, .wwi ownership, symbols, artifacts, storage, or diagnostics. Repeated occurrences of the same exact identity remain valid and deduplicate normally.
An executable package is one declared package main and containing a fn main; path and directory spelling do not classify commands. Within a package declared main, a package-scope declaration named main must be a function: let, const, def, and type forms reject as cannot declare main - must be func and do not enter package scope. The restriction depends only on the declared package name. A package with any other declared name may use or export main regardless of its dotted import path, path leaf, physical directory, or selection role. WW retains its established program-entry ABI, so a function main may carry WW's supported arguments and result; this rule does not adopt Go's source signature. An ordinary import of a package declared main is rejected, except for the toolchain's colocated external-test wiring.
For ww build, an explicit -o names an output directory when ordinary stat reports an existing directory (following symlinks) or its spelling ends in /. This classification is independent of whether one or many package roots were requested. Each selected command is published beneath that directory using the final component of its requested contextual import name, with the selected local directory leaf as the fallback when no such identity exists. Independently selected non-main roots load but do not enter the action list; non-main command dependencies retain ordinary actions, and a selection containing no command rejects before tools or output creation. A raw command-line .ww source uses its source basename without .ww, and a raw non-main source is the same no-command rejection. Missing trailing-slash hierarchies are created transactionally from 0777, filtered by the caller's umask. Loading and package/import diagnostics precede no-main, derived-path, duplicate-destination, implicit-default, and other output preflight; all of those checks precede creation. Producer failure or interruption preserves existing directory contents and removes only request-created prefixes and stages. A non-directory output retains the single-product file/archive rule. If a lone command's synthesized default basename already names a directory, loading and graph validation complete and the build rejects before tools without changing that directory; a non-main package synthesizes no default public output. Output paths and directory metadata never become package, import, graph, action, symbol, artifact, .wwi, or persistence identity.

Every caller-visible build installation checks its destination after all applicable compile, assemble, archive, and link producers finish. Ordinary stat follows symlinks. An existing directory rejects as ww: build output "PATH" already exists and is a directory; an existing nonempty regular file rejects as ... is not an object file unless its leading bytes identify a Go 1.26.5 object/output form. The recognized table is archive, ELF, Mach-O, PE, Plan 9, WASM, and XCOFF magic; WW additionally recognizes its compiler-owned //ww:module interface prefix. An absent path, an empty regular reservation, or a non-directory non-regular path may be replaced. A published non-main archive and its .wwi sidecar are checked as one WW request transaction, so arbitrary caller text in either destination preserves both old outputs and the committed persistent generation. This safety check is output disposition only: it does not enter package/import loading, graph or action identity, artifact bytes, or invalidation. Assembly-only -S retains the directory form's command-action selection and no-main rejection, but it reaches no install action: destination length, duplicate publication names, implicit destination collision, and output parent creation are therefore inapplicable.
A newly published ww build command is created with permission 0777 filtered by the invoking process's umask. A newly published non-command archive, and the adjacent WW interface required to consume it, use 0666 filtered by that same umask. These inode permissions are output metadata: they do not enter canonical package or action identity, artifact bytes, import binding, symbols, .wwi contents, or persistent invalidation. A warm build therefore reuses unchanged semantic actions while refreshing the caller-visible output with the current invocation's mode. Assembly-only builds publish no executable or archive.
On the Unix target, exact -o /dev/null is a build-output discard request, not an ordinary output filename. Package loading, import validation, graph construction, compilation, assembly, archiving, command linking, failure, and persistent invalidation proceed normally; only installation of a caller-visible command, archive, interface, default basename, or adjacent scratch tree is omitted. One such request may select any number of command and non-command packages, and an empty recursive match is successful after its normal warning. -S likewise needs no caller workdir merely to retain discarded assembly. Raw-file and directory roots use the same rule. A non-exact spelling is an ordinary output and keeps all established fan-out, rejection, permission, scratch, and publication behavior. Output discard is request metadata; it does not change package, import, graph, action, artifact, symbol, .wwi, or persistence identity.
Only names marked export (§5) are visible across module boundaries.

Import paths remain unquoted and dotted. Grouped imports, quoted import paths, and dot imports are not implemented. _ is reserved here for the blank form; it is not an ordinary alias.

5. Declarations

TopDecl = [ "export" ] ( FnDecl | TypeDecl | LetDecl | ConstDecl
                       | DefDecl ) .

5.1 Functions

FnDecl = "fn" ident "(" [ Params ] ")" Result "=" Block .
       | "fn" ident "(" [ Params ] ")" Result ";" .   // bodiless: extern/FFI
Params = Param { "," Param } .
Param  = ident ":" Type | ident ":" Type "..." | "..." .
Result = Type | "void" .

The = between signature and body is required. A bodiless fn (; terminator) declares an external symbol (C FFI); the bare ... C-style variadic is legal only on a bodiless declaration (§6.3).

At package scope, exactly fn init() void = Block; declares a special package initializer. It must have a body, no parameters, no result, no export, and no attribute. Multiple init declarations are permitted and retain owner-file and source order. init is not inserted into ordinary package scope: it cannot be called as init(), selected as pkg.init, exported, or used by another kind of declaration. Package-variable initialization completes before these functions run.

In a package whose declared name is main, only a function declaration may claim the package-scope name main. A rejected non-function declaration is not installed and cannot satisfy the executable entry. This is independent of canonical import identity and physical location. The accepted function shape continues to use WW's entry ABI, including its supported argument and result forms.

5.2 `let`

LetDecl = "let" ident ":" Type [ "=" Expr ] ";" .

A mutable binding. Without an initialiser, package-level storage is zero-backed; an uninitialized function-local binding retains the existing uninitialized-local rule. A static qualifier (static let …) gives function-local storage static lifetime.

For a package-level let with an initializer, values representable by the static-data emitter are installed statically. Every other otherwise valid initializer is evaluated exactly once at runtime. Checked references through package functions contribute variable-dependency edges; dependencies precede dependents, and source declaration order breaks ready ties. Initialization cycles are errors. Runtime package lets execute after imported package tasks and before the package's init functions. const and def are not broadened by this runtime path.

5.3 `const`

const introduces an immutable storage binding of the same declaration shape as let; assignment to it is rejected. At package scope its initializer stays within the existing static-data forms. A const is not entered into the runtime package-variable schedule described above; use mutable let when an otherwise valid initializer requires runtime evaluation.

5.4 `def`

DefDecl = "def" ident ":" Type "=" ConstExpr ";" .

A compile-time constant definition, typically at module level (def AF_INET: i32 = 2;). Hare's def.

5.5 `type`

TypeDecl = "type" ident "=" Type ";" .

5.6 Visibility

export on a top-level declaration makes the name visible to importing modules. Unmarked names are module-private.

5.7 Attributes

@name prefixes a declaration with an attribute. The defined attribute is @test: it marks a function as a test (§10).

6. Functions and calls

6.1 Blocks and results

A function body is a Block (§7). The body's value is its result; an explicit return Expr; returns early.

6.2 Calls

Call = Expr "(" [ Args ] ")" .
Args = Arg { "," Arg } [ "..." ] .

Arguments are passed by value (a str/[]T/struct copy is a shallow header/field copy). Aggregates larger than a register use the aggregate/sret ABI.

6.3 Variadic

Two distinct variadic forms:

Hare-style args: T... — the callee receives a []T. A call either gathers loose arguments (fmt.println(1, "x")) or forwards an existing slice with the spread xs....
C-style bare ... — only on a bodiless (extern) declaration, for calling C variadic functions across the FFI. f32 arguments in the variadic tail are promoted to f64 per the C ABI.

7. Statements

Stmt = Block | LetDecl | ConstDecl | Assign | If | For | Switch
     | Match | Return | Break | Continue | Defer | Yield | ExprStmt .
Block = "{" { Stmt } "}" .

7.1 Assignment

Assign = Expr AssignOp Expr ";" .
AssignOp = "=" | "+=" | "-=" | "*=" | "/=" | "%="
         | "&=" | "|=" | "^=" | "<<=" | ">>=" .

The compound forms apply to scalar, indexed, and pointer-field targets.

7.2 `if`

If = "if" "(" Expr ")" Block [ "else" ( If | Block ) ] .

7.3 `for`

For = "for" "(" Expr ")" Block                          // condition
    | "for" "(" LetDecl Expr ";" Expr ")" Block          // 3-clause
    | "for" "(" "let" ident ".." Expr ")" Block .         // range

The 3-clause form is for (let i: size = 0; i < n; i += 1) { … }. The range form iterates the elements of a slice or array (for (let x .. xs) { … }). break and continue apply to the nearest enclosing loop; continue runs the post-clause / advances the range.

TODO(spec): does the range form bind the element or an index, and is the bound name a copy or a view? Confirm against the checker's N_FORRANGE lowering; the continue-post behaviour was a fixed miscompile (project memory #138), so the desugaring is worth stating precisely.

7.4 `switch`

Switch  = "switch" "(" Expr ")" "{" { SwCase } "}" .
SwCase  = "case" ExprList ":" { Stmt } | "case" ":" { Stmt } .

Value-matching on a scalar. A case may list comma-separated values; case: is the default arm. Arms do not fall through.

7.5 `match`

Match    = "match" "(" Expr ")" "{" { MatchArm } "}" .
MatchArm = "case" [ "let" ident ":" ] Type "=>" ArmBody
         | "case" "=>" ArmBody .
ArmBody  = Stmt | Block .

Discriminates a tagged union (§3.7). case let v: T => binds the narrowed value to v; case T => matches without binding; case => is the default. A match used as an expression produces a value via yield (§7.7) from each arm.

7.6 `return`, `break`, `continue`, `defer`

return [Expr]; break; continue;. defer Stmt; schedules a statement to run when the enclosing block exits.

7.7 `yield`

yield Expr; produces the value of an enclosing block- or match- expression.

8. Expressions

8.1 Operands

Identifiers, qualified names (mod.name), literals (§2.7), struct/array/ tuple literals (§8.5), and parenthesised expressions.

8.2 Operators and precedence

From loosest to tightest binding:

||
&&
==  !=  <  <=  >  >=
|   ^
&
<<  >>
+   -
*   /   %
unary:  -  !  ~  &(addr-of)  *(deref)
postfix: call()  index[]  slice[:]  field.  ?  !

&e takes the address of an addressable operand; *p dereferences.

TODO(spec): the precedence ladder is transcribed from the parser's bprec (parse.ww); verify each tier against cmd/wcc/parse.c and state associativity per tier. ww has no generics — no type parameters anywhere in the grammar; call this out explicitly once confirmed.

8.3 Casts, `as`, `is`

Cast = Expr ":" Type .          // type ascription / conversion
As   = Expr "as" Type .         // tagged-union narrowing
Is   = Expr "is" Type .         // tagged-union variant test (bool)

expr: T converts/ascribes (len(xs): size, t.line: i64). An integer cast does not silently truncate beyond the target width. e as T narrows a tagged union to variant T; e is T tests membership.

8.4 Index, slice, field, pseudo-fields

a[i] indexes an array/slice/str (index operand must be an integer). a[lo:hi] produces a sub-slice. e.f selects a struct field or a qualified name. .ptr, .len, .cap read the header words of a str or slice.

8.5 Composite literals

StructLit = TypeName "{" [ FieldInit { "," FieldInit } ] "}" .
FieldInit = ident ":" Expr .
ArrayLit  = "[" [ Expr { "," Expr } ] "]" .
TupleLit  = "(" Expr "," Expr { "," Expr } ")" .

S{} is the zero value of S.

8.6 Builtins

size(T) (type size), len(x) (str/slice/array length), alloc(v)! (heap-allocate a value, yield *T; §9), append(s, x) (grow a slice), delete(…), abort(msg) (terminate; type never).

alloc takes a value: alloc(T{…})!, alloc(T{})!, or alloc(expr)!. Bare alloc(T)! is invalid (it parses T as a value reference) (§12). There is no free: ww has no GC and no manual reclamation in compiler-side code — allocation lives until process exit (§12). lib/* modules that genuinely own a heap buffer free it explicitly via os.free.

9. Errors

ww has no exceptions. A fallible operation returns a tagged union whose error variants carry the !T mark (§3.7). Two postfix operators consume them:

expr? — if expr is an error variant, return it from the current function (propagate); otherwise yield the success value.
expr! — if expr is an error variant, abort; otherwise yield the success value.

Error variants are ordinary sentinel types in the union (strconv.invalid, strconv.overflow, …), discriminated with match. This mirrors Hare's error idiom, spelled with ww's ! tag.

10. Tests

A function marked @test is a test, run by the test harness, not part of the program. Two placements, following the Go foo / foo_test model:

White-box, in-package: a colocated file package <mod>; with @test functions, for testing unexported internals.
Black-box, external: package <mod>_test; with import <mod>;, exercising only the public surface.

@test fn adds() void = {
	if (1 + 1 != 2) { abort("math broke"); };
};

Production, production-plus-white-box-test, external black-box-test, test support, recompiled-for-test dependencies, and generated test main are distinct package actions. One canonical selected directory owns one test product and one generated main, which imports every applicable same-package and external target and produces one binary/result. The external action's import of the package under test binds to the augmented white-box action when it exists; affected transitive importers are copied and rewired so ordinary and augmented package state do not coexist in the linked closure.

A non-function package-scope main in production or same-package test source of a package declared main is a package-checker failure. Generated test-main ownership does not hide or replace it: the product is not linked or executed, no test accounting or package ok result is emitted, and ordinary build/test failure presentation and rollback apply.

Those variants are action distinctions over exact package representatives, not new ordinary package identities for case-fold comparison. Production, same-package test, external test, and recompiled copies of one exact canonical package therefore do not collide with each other. Their selected source units remain separate, while the directory-owned filename preflight spans the production/internal/external selections applicable to that test product.

Before a test function runs, the one generated product initializes its exact effective graph dependency-first and once per canonical action. Imports, runtime lets, and init declarations found only in *_test.ww never enter an ordinary production build. Test-only same-package, external-package, and valid mixed directories are accepted from their selected test files. A directory with no selected test file validates ordinary production but creates no test support, generated main, link, binary, captured runtime result, or process. The coordinator reports that successful validation exactly as ? <package> [no test files]\n.

Every test-bearing directory product links one request-private runnable. -c retains an executable copy and suppresses its execution; -o retains a copy at the named destination and still executes unless -c is present. With no explicit output, -c writes <import-leaf>.test in the invocation directory. An output ending in / or naming an existing directory receives that basename and may have missing parent directories created. One non-directory output may name only one package. Multiple packages whose visible import leaves would produce the same test-binary name reject before tools or output creation; exact /dev/null is the discard exception. It keeps the ordinary private link and, unless -c is present, the ordinary execution, but installs no retained copy. The raw single-file compatibility route uses the same private-output rule and never treats /dev/null as an explicit artifact or adjacent-scratch stem. Declared package names, test variants, source filenames, physical directories, output paths, and retained binary names remain presentation or loader metadata and do not become canonical package or action identity.

The retained file is an executable, byte-identical copy of the private runnable. A compile-only retained binary joins package artifacts and statuses in the request-wide atomic publication transaction. Build, link, stage, or install failure preserves old destinations and removes temporary stages and invocation-created output prefixes. For a running -o request, the private binary executes first. Only a successful run enters the guarded install action; a failed, signalled, timed-out, interrupted, or unstartable run publishes no new copy and preserves any prior destination. The post-run guard uses the same directory/nonempty-regular/object-magic rule as ww build. Successful products in a multi-package running request install independently; their visible result order remains package order. A no-test product publishes no binary and does not create a directory solely for one. -w may persist the unchanged semantic actions for either -c or running retention without changing publication identity or introducing a test-result cache.

When ww test executes a directory-owned product, the child process working directory is that product's canonical absolute physical package source directory. Its per-run environment has exactly one effective uppercase PWD, with that same value. The physical source directory is execution context, not canonical package identity: it does not enter dotted identity, import binding, action identity, mangled symbols, .wwi, artifacts, or persistence keys. Production, internal-test, external-test, recompiled-for-test, support, and generated-main actions in the product share the one process context while remaining separate actions. Reachable dependency initialization consequently observes the tested product's directory; a dependency tested as its own product observes its own directory.

Every test binary actually started by ww test receives one effective uppercase PATH whose first element is the canonical absolute directory of the selected WW driver. An absent or empty caller PATH yields only that directory; a nonempty effective caller value follows it after :. Normal duplicate PATH= entries collapse to that one value, using the caller's first effective value as the suffix. This applies to directory products, their dependency initialization, filters and list mode, running retained tests, and the raw single-file compatibility route. It does not apply to build tools, ww build, ww run, compile-only or assembly-only test requests, no-test products, or later direct execution of a retained binary.

An executed directory product otherwise receives the caller's original Unix environment, not the build plan's tool environment. The snapshot keeps the first occurrence of each normal case-sensitive key=value, omits later normal duplicates and raw empty entries, and retains every nonempty malformed entry in order. It then applies only the effective PATH above and the package PWD. Consequently caller LC_ALL, TMPDIR, empty-valued variables, case-distinct keys, and arbitrary variables remain visible to production or test-only initialization and test bodies. The compiler, assembler, archiver, linker, and coordinator scratch continue to use their separate build-plan locale and temporary directory.

The same coordinator-executed product reads standard input from the null device. Its first read observes EOF regardless of the terminal, pipe, or file connected to the invoking command. Each parallel product owns a separate null descriptor, and production or test-only dependency initialization, filtering, listing, no-match execution, failure, and timeout all retain that boundary. Standard input is runtime process metadata and contributes no package, action, artifact, or persistence identity.

The product's standard output and standard error refer to one product-local capture. Bytes from either descriptor retain the order in which their writes reach that shared open output, and the coordinator emits the completed capture on its standard output. Immediately before an existing directory-product ok or run-status FAIL trailer, a nonempty capture whose final byte is not newline receives exactly one newline; an empty or already newline-terminated capture receives none. The capture itself is unchanged, and raw single-file or later direct retained-binary execution has no coordinator trailer and no such separator. A runtime failure or child-setup failure appends the product status diagnostic to standard output as well. Loader, source, compiler, assembler, linker, and other build failures retain their diagnostic standard-error channel; build-action stdout and stderr remain separate. Parallel products own independent captures and are still emitted in canonical product order.

List mode starts the same directory-owned product and therefore performs its ordinary package initialization, but it does not start any selected test child. The harness emits exactly one newline-terminated qualified name for each test selected by the command's filters, in descriptor order. If the selected set is empty, the harness emits no list payload, warning, or accounting and returns success; the coordinator still emits the ordinary successful package result without a no-tests suffix. Ordinary non-list execution is distinct: a valid filter selecting no tests writes exactly testing: warning: no tests to run\n through the harness's standard error, preserves the discovered/selected/started/completed accounting and successful exit, and causes a successful directory package result to end in [no tests to run]. The coordinator recognizes that exact warning only at capture byte zero or after a newline. Directory execution combines the warning and accounting in its product-local capture; a retained binary invoked directly keeps the warning on standard error, its accounting on standard output, and has no coordinator package result.

After those ordered results, an ordinary explicit ww test request that records an attributable setup, build, or execution failure emits exactly one standalone FAIL\n on standard output. The line is command-owned: it follows even a later successful package result, and a single explicit directory, recursive or dotted directory, or raw source file is sufficient. Runtime nonzero exit, signal, timeout, or executable-start failure all set that status. Filters, list mode, and the private execution of a retained test keep the same rule. Bare implicit current-directory ww test, -c, -S, build requests, CLI usage/shape or output preflight, publication-only failure, capture-only failure, cleanup-only failure, and later direct execution of a retained binary do not emit it. The marker changes no capture, diagnostic, executable, package/import/action identity, publication decision, or persistent byte.

The coordinator does not change its own cwd or environment. Parallel products receive independent child environments and each uses its own source directory. Relative ordinary files, testdata, and writes resolve there for every executing filter or list path. ww build, directory ww test -c (including -c -o), and a no-selected-test directory execute no test child and receive no execution-directory effect. A published test binary invoked directly, and the raw single-file compatibility route, inherit the user's invocation cwd and three standard descriptors; no package directory, input, or output policy is embedded or forced by the binary. Directly invoked retained binaries inherit the entire caller environment. Raw single-file tests preserve every caller environment field except the test-only PATH transformation above.

11. Concurrency (reserved)

The headline +CSP surface — chan T, proc/spawn, channel send/ receive <-, and the alt select construct — is tokenised (chan, proc, <-, ->) but not yet given semantics. This section is written when the feature lands; until then, use of these tokens is rejected by the checker.

Meta-ruling (normative). CSP is the sanctioned departure from the Hare-fidelity rules (CLAUDE.md rules 5 and 9) — Hare has no channels, and the +CSP mandate is the project's reason to add them. The concurrency surface therefore cites the Newsqueak → Limbo → Go lineage, not Hare, and that citation is correct by construction, not a rule-9 violation. The design of record is .ai/csp-design-input.md (library-first: spawn, unbuffered bidirectional chan T via mkchan(T), <- send/recv, recv → (T | closed) error idiom, alt as v2). See §12 entry 9.

TODO(spec): fill in syntax + semantics (channel construction, send/recv typing, spawn arg-copy ABI, alt arm grammar, the closed sentinel) once the MVP lands and the USER meta-ruling is explicitly on record.

12. Divergences from Hare (normative)

ww follows Hare's semantics and lib/ API shapes (CLAUDE.md rules 5, 9) except for the deliberate, USER-blessed departures below. Each is normative; the cited reference is the rationale of record.

int/uint are a machine word (8 bytes), not Hare's 32-bit int. All limit constants derive from size(int); size/uintptr are 8 bytes. Ref: project memory "int is a machine word"; USER 2026-05-26.
Tuples use 8-byte slots, not Hare's packed layout. size((u32, u32)) == 16. Routed through the type table only. Ref: USER 2026-06-04 tuple slot ruling; parity task #60.
Nullability is the union (*T | void), not a nullable qualifier. Plain *T is non-null; the union fold is one flag, zero-cost for the non-null path. Ref: USER 2026-05-26 nullable-kept ruling.
No _unsafe suffix convention. ww is unmanaged (no GC, no safe baseline), so Hare's _unsafe axis does not apply. bytes→str is the pure reinterpret strings.frombytes (Hare's fromutf8_unsafe); UTF-8 validation is opt-in at the IO source via utf8.validate, not wrapped per construction. Ref: CLAUDE.md rule 9; ref/hare/strings/ utf8.ha.
The compiler frontend is one package lib/ww/syntax (tok + lex + ast + sym + typ + parse), a Go-over-Hare consolidation of Hare's {ast, lex, parse} split. Its single consumer is the wcc backend. The internal data shapes still mirror ref/hare/hare. Ref: CLAUDE.md rule 6; USER #74.
Tests use the Go foo / foo_test model (§10): external black-box package <mod>_test; plus in-package white-box @test files. ww hard-rejects self-import. Ref: CLAUDE.md rule 9; task #16.
No arena / no free in compiler-side code. Per-node alloc(value)!; allocation is reclaimed at process exit. alloc takes a value (alloc(T{…})!), never a bare type. Ref: project memory "drop amalloc", "Phase 0 closed", "alloc syntax form".
The checked AST carries a per-node type_ stamp. Hare's parse AST is untyped and harec keeps a separate checked AST in C; ww stamps node.type_ in place as a checker invariant. Ref: project memory "Hare AST has no per-expr result".
CSP (§11) is the sanctioned departure from rule-5/9 Hare fidelity — Hare has no channels. The surface cites Newsqueak/Limbo/Go, not Hare. Ref: .ai/csp-design-input.md; pending explicit USER meta-ruling.
The two compiler stages emit byte-identical asm for the same input (CLAUDE.md rule 10); when inference power differs, the richer stage is aligned down to the leaner one. This is a project invariant, listed here because it is the contract this spec's "the text is the bug if stages disagree with it" rule rests on.

Appendix A. Anti-rot

This spec is hand-maintained. To keep it from drifting against the parser, every fenced ```ww example is intended to be extracted and compiled through w6c, asserting it parses (or rejects, for negative examples), wired as a make spec-check dependency of make test. The EBNF productions are not auto-verified; the compiling examples are the drift alarm. (Check not yet implemented; filed as a follow-up.)

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